Akademik Veri Yönetim Sistemi
JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS, cilt.275, sa.3, ss.1656-1662, 1995 (SCI-Expanded)
Previous studies showed that alpha, adrenoceptor-mediated contractile responses change with age in the rat aorta, becoming more sensitive to Ca++ channel blockers and less sensitive to chlorethylclonidine (CEC), suggesting a change in the alpha(1) adrenoceptor subtypes that are present. In this study, alpha(1) adrenoceptor density and alpha(1) adrenoceptor subtypes were measured in the Fischer 344 rat aorta during aging. Aortic alpha(1) adrenoceptor densities, determined by saturation binding of 2-[beta-(4-hydroxy-3-[I-125]iodophenyl)ethylaminomethyl] tetralone ([I-125]-HEAT), were 47, 41 and 45 fmol/mg protein in 1-, 6- and 24-month-old rats, respectively. The noncompetitive antagonist CEC completely blocked [I-125]-HEAT binding in aortas from 1-month-old rats but inhibited binding only partially in aortas from older rats. Two binding sites were detected for norepinephrine and for WB4101 in all ages. The low-affinity constants for WB4101 (31-51 nM) were consistent with those for the alpha(1b) adrenoceptor subtype, and this binding site decreased with age. The high-affinity constant for WB4101 (1.4 nM) in I-month-old aorta was consistent with that for alpha(1d) adrenoceptor subtype, whereas the high-affinity constants (0.03 nM) in 6- and 24-month-old aortas were consistent with those for the alpha(1a) adrenoceptor subtype, At least three alpha(1) adrenoceptor subtypes appear to be colocalized in the rat aorta, so the binding affinities may reflect binding to more than one subtype. This makes it difficult to identify definitively the subtypes based on their radioligand binding characteristics. To provide a more definitive determination of the subtypes with age, the levels of the alpha(1b) and alpha(1d) adrenoceptors and their mRNAs were measured by immunoblotting and Northern hybridization, respectively. Expression of the alpha(1b) adrenoceptor decreases with age in the rat aorta as evidenced by decreased immunoreactivity and decreased mRNA. In contrast, alpha(1d) adrenoceptor immunoreactivity and mRNA do not change with age. These results show that the alpha(1b) adrenoceptor is the major subtype in young aortas, and that the expression of the alpha(1b) adrenoceptor subtype decreases with age, whereas the expression of the alpha(1d) adrenoceptor subtype does not change with age, Radioligand binding experiments also revealed the presence of the alpha(1a) adrenoceptor (insensitive to CEC and with very high affinity for WB4101) in the Fischer 344 rat aorta and an increase in its expression with age.